In the manufacture of glass-metal electrical components used, for example, as an ignitor or actuator containing pyrotechnic material electrically detonated to perform work (e.g., to drive a piston), it is essential that various metal and glass parts thereof be accurately assembled and fused since any slight mis-alignment of parts may make the component inoperative.
One type of glass-metal electrical component that can be assembled with the present invention comprises a metallic shell formed with upper and lower cylindrical recesses communicating with each other through longitudinal passages formed within the shell body. A set of metal pins extending through the passages have upper and lower ends respectively terminating within the upper and lower recesses. These pins are embedded within glass, providing insulation between the shell and pins. Upper ends of the pins are connected to bridge wires in contact with pyrotechnic material sealed within the upper recess. Lower ends of the pins which are joined to cylindrical concentric metal sleeves are fixed within the lower recess with insulative glass to establish electrical feed-throughs which may be connected to a suitable plug for detonating the device.
Assembly of the aforesaid electrical component initially requires placement of the metal sleeves with or without welded pins within a fixture having nesting and stop surfaces for aligning the sleeves in spaced, concentric locations. Thereafter, the metal shell is fitted to other nesting surfaces within the fixture so that the metal sleeves are positioned within the lower recess spaced from the shell walls. A glass pre-form is then seated within the upper recess. The fixture and assembled component are then placed within a furnace, heated to melt the glass pre-form. The melted glass flows by gravity through the passages into the lower recess to provide an insulative seal between the shell and sleeve/pin configuration throughout the shell body.
Because of surface tension between the various component parts and the glass, the viscosity of the glass, and temperature limitations, complete glass flow often cannot occur, resulting in formation of a poor or incomplete glass-to-metal seal. Frequently, air pockets are trapped within the glass seal, reducing the superior insulating effect of the glass.
Weighted plungers have been used to improve the gravity flow of melted glass during the forming process. However, depending upon the spacing between the sleeve/pin configuration and the shell body, or employment of this sealing technique in other structures where glass or glass-ceramic-to-metal seals are necessary, there are circumstances where plungers do not allow the formation of reliable seals.